A permanent magnet motor (hereinafter, simply “motor” unless otherwise particularly specified) has been known as a high-efficiency motor for the following reasons. As compared with an induction motor conventionally widely used in various fields, the permanent magnet motor does not require an excitation current because a magnetic flux by a permanent magnet incorporated in a rotor is established, and the permanent magnet motor does not generate a secondary copper loss because a current does not flow to a rotor conductor unlike in the induction motor. Although induction motors have been conventionally used for electric vehicles, in recent years, application of a permanent-magnet synchronous motor to electric vehicles has been examined so as to improve efficiency, to generate a large output in a reduced size, and to simplify cooling structures.
Generally, in an electric vehicle that runs while having a formation that has a combination of a plurality of vehicles, which have motors and drive control devices incorporated thereon, even when a motor connected to a drive control device in a part of the vehicles becomes inoperative because the drive control device stops functioning by generating a short-circuit defect during a running of the electric vehicle, for example, the electric vehicle can still continue running by other sound drive control devices and other sound motors. As a result, because the motor connected to the defective drive control device is kept being driven from a wheel side, a short-circuit current by an induced voltage of the motor continues to flow to a defective portion (a short-circuited position) of the drive control device having the short-circuit defect.
Therefore, when this state is left as it is, there is a risk of further worsening the damage of the defective portion of the drive control device by a short-circuit current, heat generated by the short-circuit current and the like, and causing heat generation and burning loss of the defective portion or the motor, and this is undesirable.
To deal with such cases, there is disclosed a method in which a motor open contactor as a motor-side switch unit that electrically separates a connection between an inverter and a motor is provided, so as to prevent worsening of the damage of an inverter by an induced voltage of a motor when a defect occurs in the inverter within a drive control device that drive-controls the motor when an electric vehicle is running. In this method, when a controller detects a defect of the inverter, the controller turns off the contactor and separates the inverter from the motor (for example, Patent Literature 1).    Patent Literature 1: Japanese Patent Application Laid-open No. H8-182105